Poly(ethylene ether carbonate) polyols can be modified by chemical reactions with polyglycol modifiers under conditions of elevated temperatures and reduced pressures, where diethylene glycol is removed as distillate. Molecular weight builds in a predictable manner and can be controlled by the amount of distillate removed. A reaction pathway for this transformation is proposed. It is important that the modifier be stable to reaction conditions and have a higher boiling point than diethylene glycol. The process has been characterized by following the distillate formation and composition as a function of reactor temperature and product molecular weight build. The product has been characterized by molecular weight, CO2 content, carbon–13 NMR and viscosity/temperature relationships. Carbon–13 NMR of the carbonyl and methyl regions of the spectra are particularly useful for following the chemical changes. A new family of modified poly(ethylene ether carbonate) polyols has been prepared which was not available with prior technology. The modification procedure permits the control of properties such as modifier type and content, molecular weight, CO2 content, viscosity, Tg, density, etc., in the resultant polyol. Since poly(ethylene ether carbonate) polyols have predominantly polyester polyol properties, modification with polyether polyols can be used to adjust properties. These polyols are useful in polyurethane applications.